1. Field of the Invention
The present invention relates generally to a waveguide element, a waveguide branch or coupler element and a waveguide integrated circuit. More specifically, the invention relates to a waveguide discontinuity, an extremely high frequency wave integrated circuit, an optical integrated circuit, a quantum electronic device or so forth using the waveguide discontinuity.
2. Description of the Related Art
Associating with increasing optical elements or increasing of kinds of the optical elements to be employed in an optical communication system, cost and mounting load for overall system is becoming significant. In such circumstance, necessity for optical integrated circuit [PIC (Photonic Integrated Circuit)], on which a plurality of optical elements are integrated on a single substrate in monolithic manner, for realizing a specific finction, has been put forward.
On the other hand, for the purpose of extreme enhancement of electronic circuit element, development and research has been started for a quantum electronic device seeking for new possibility of quantum effect as a wave of electron (de Broglie wave) employing a nanometer class fine fabricating technology.
In such element, function of a branch or coupler for coupling between respective functional elements becomes more important, as well as enhancement of performance of respective of individual functional elements. Particularly, for realizing higher two-dimensional package density, acute waveguide discontinuous element, such as a right angle bending (L-shaped bending) or a branching (T-shaped branching) is inherent for enhancing freedom in layout of respective functional elements to be comparable with an electrical integrated circuit.
However, in order to strictly analyze behavior of electron or so forth in a fine structure of extremely high frequency wave, a submillimeter wave and further nanometer class, it is necessary to handle these as a wave. Naturally, it is inevitable to cause reflection and radiation to be a primary cause of bending of the waveguide or loss in discontinuity.
As a known technology for restricting excessive loss in acute waveguide discontinuity, a tapered structure gradually deforng cross-sectional shape of the waveguide over a sufficiently long region in comparison with a wavelength, has been widely employed. However, this inherently increases the overall length of the waveguide integrated circuit. As a result, various problems to overcome, in constrainint layout freedom of respective elements, lowering of yield percenteedegration degree, increasing of absorption loss, have been encountered.
In order to arbitrarily bend propagating direction of a light beam, it is a typical method to employ a mirror (reflection mirror). A typical waveguide bending element widely studied for the purpose of application to optical integrated circuit or so forth, is a corner reflector.
In the foregoing conventional optical integrated circuit, the acute waveguide discontinuous element, such as right angle bending (L-shaped bend)or branching (T-shaped branch) is inherent for enhancing freedom in layout of respective functional elements to be comparable with an electrical integrated circuit.
While the reflection mirror is employed for bending propagating direction of the light beam at an arbitrary angle, the mirror functions as an ideal reflecting plane only when it is sufficiently larger than the wavelength of a beam form wave propagating in a multi-mode space. In other words, the mirror of the size substantially equal to the wavelength or smaller merely serves as scattering body arranged on a path, for the wave.
The corner reflector as the waveguide bending element does not effectively achieve function as a pure reflection mirror as long as it is mounted in a single mode waveguide. The reason why the corner reflector appears to behave for being the path of the light is that the single mode waveguide on the output side picking up a diffracted wave generated by scattering by the cornering reflector barely encloses the diffracted wave in lateral direction (direction perpendicular to a propagation axis).
As set forth above, as the structure for realizing acute waveguide discontinuity in the optical integrated circuit, the structure which is applicable for practical use, is not present.
The present invention has been worked out for solving the problems set forth above. Therefore, it is an object of the present invention to provide a waveguide element, a waveguide branch or coupler element and a waveguide integrated circuit, which can realize acute waveguide discontinuity, such as a right angle bending (L-shaped bending) or branching (T-shaped branching).
According to the first aspect of the present invention, a waveguide element having a waveguide discontinuity including at least one of bending and intersection of straight waveguide propagating a wave, comprises:
first and second straight waveguides forming at least one of bending and intersection; and
a leaky wave propagation region establishing a leaky wave coupling of the wave propagating through said first and second straight waveguides at substantially equal rate.
According to the second aspect of the present invention, a waveguide branch or coupler element having a waveguide discontinuity including at least one of bending and intersection of straight waveguides propagating a wave for branch or couplersaid wave, comprises:
first, second and third straight waveguide forming at least one of said bending and intersection;
a first leaky wave propagation region establishing a leaky wave coupling of said wave propagating through respective of said first and second straight waveguides at substantially the same rate; and
a second leaky wave propagation region establishing a leaky wave coupling of said wave propagating through respective of said first and third straight waveguides at substantially the same rate.
According to the third aspect of the present invention, a waveguide integrated circuit, in which a waveguide element having a waveguide discontinuity including at least one of a bending and an intersection of straight waveguide propagating a wave, is integrated on a substrate, comprises:
first and second straight waveguides forming at least one of bending and intersection; and
a leaky wave propagation region establishing a leaky wave coupling of the wave propagating through said first and second straight waveguides at substantially equal rate.
An object of the present invention is to realize an acute waveguide discontinuity by restricting an excessive loss caused by reflection or radiation of wave in the waveguide bending or intersecting the straight waveguides propagating a general wave including an electromagnetic wave, such as an extremely high frequency wave, a light and the like or a de Broglie wave of electron or the like.
The waveguide element includes two single mode waveguides have center axes extending in longitudinal directions and intersecting at an intersection angle xcex8 (0 less than xcex8xe2x89xa690xc2x0) and have effective propagation wavelength xcexG, and a leaky wave propagation region is a region of effective propagation wavelength xcexL having two edges located close to each other along the longitudinal direction of respective of two single mode waveguides so as to establish leaky wave coupling of the wave propagating through the single mode waveguides at substantially the same rate.
The two single mode waveguides and said leaky wave propagation region are single mode in the direction perpendicular to the plane including two single mode waveguides and in symmetric relationship with respect to a particular plane perpendicular to a plane including two single mode waveguides and including a straight line equally dividing a supplementary angle (xcfx80xe2x88x92xcex8) of an intersecting angle xcex8 (0 less than xcex8xe2x89xa690xc2x0) of the two single mode waveguides.
A relationship expressed by:
cos(xcex8/2)≈xcexL/xcexG
is substantially established to make various amounts of the effective propagation wavelength xcexG and the effective propagation wavelength xcexL to be sufficiently small, and two single mode waveguides are coupled via the leaky wave propagation region.
An object of the present invention is to provide a waveguide branch or coupler element, which realizes an acute waveguide discontinuity by restricting an excessive loss caused by reflection or radiation of wave in the-waveguide bending or intersecting the straight waveguides propagating a general wave including an electromagnetic wave, such as an extremely high frequency wave, a light and the like or a de Broglie wave of electron or the like.
The waveguide branch or coupler element includes first single mode waveguide of effective propagation wavelength of xcexG, second and third single mode waveguides of effective propagation wavelength of xcexG which intersect first single mode waveguide at an intersection angle xcex8 (0 less than xcex8xe2x89xa690xc2x0), and first and second leaky wave propagation region of effective propagation wavelength of xcexL.
These are single modes in perpendicular direction including a plane including first and second single mode waveguides and are in symmetric relationship with respect to a particular plane perpendicular to a plane including said first and second single mode waveguides and including a straight line equally dividing a supplementary angle (xcfx80xe2x88x92xcex8) of an intersecting angle xcex8 (0 less than xcex8xe2x89xa690xc2x0) of said first and second single mode waveguides.
A relationship expressed by:
cos(xcex8/2)=xcexL/xcexG
is substantially established, the first and second single mode waveguides are mutually coupled via the first leaky wave propagation region, and similarly, the first and third single mode waveguides are mutually coupled via the second leaky wave propagation region.
An object of the present invention is to provide a waveguide integrated circuit, in which the waveguide element realizing an acute waveguide discontinuity by restricting an excessive loss caused by reflection or radiation of wave in the waveguide bending or intersecting the straight waveguides propagating a general wave including an electromagnetic wave, such as an extremely high frequency wave, a light and the like or a de Broglie wave of electron or the like, is integrated on the substrate.
The waveguide element includes two single mode waveguides have center axes extending in longitudinal directions and intersecting at an intersection angle xcex8 (0 less than xcex8xe2x89xa690xc2x0) and have effective propagation wavelength xcexG, and a leaky wave propagation region is a region of effective propagation wavelength xcexL having two edges located close to each other along the longitudinal direction of respective of two single mode waveguides so as to establish leaky wave coupling of the wave propagating through the single mode waveguides at substantially the same rate.
The two single mode waveguides and said leaky wave propagation region are single mode in the direction perpendicular to the plane including two single mode waveguides and in symmetric relationship with respect to a particular plane perpendicular to a plane including two single mode waveguides and including a straight line equally dividing a supplementary angle (xcfx80xe2x88x92xcex8) of an intersecting angle xcex8 (0 less than xcex8xe2x89xa690xc2x0) of the two single mode waveguides.
A relationship expressed by:
xe2x80x83cos(xcex8/2)≈xcexL/xcexG
is substantially established to make various amounts of the effective propagation wavelength xcexG and the effective propagation wavelength xcexL to be sufficiently small, and two single mode waveguides are coupled via the leaky wave propagation region.
The present invention is to provide a method for realizing a quite effective and simple construction under a condition where the wave per se bend propagating direction in discontinuity of two single mode waveguide, center axes extended in the longitudinal direction are intersect.
A basic principle is based on quite simple principle for enabling couple of energy of the wave (coupling) between two single mode waveguides via the leaky wave or the like.
Namely, the idea is that since radiation of energy should be caused in the waveguide discontinuity to be loss in any event, and thus 100% radiation is positively caused to collect 100%.
Hereinafter, concerning construction or the like for realizing the principle, more detailed discussion will be given with explanation of physical phenomenon caused around a mirror (reflection mirror) important for understanding of the invention.
A passage xe2x80x9cideal function can be obtained only when a mirror greater than wavelengthxe2x80x9d suggests quite important and interesting phenomenon occurring around the reflection mirror. It is the fact xe2x80x9cthe condition is established around the reflection mirror at a certain condition where the wave subjecting scattering of the reflection mirror appears to be propagated with aggregated as if it is aggregated into a single beam even in a space having completely not enclosed in the lateral direction, such as a free space.xe2x80x9d
The phenomenon xe2x80x9cappearing to be propagated with aggregated as if it is aggregated into a single beam even in a space having completely not enclosed in the lateral direction, such as a free spacexe2x80x9d suggest possibility of presence of more effective means utilizing a nature of the wave other the method forcedly enclosing the wave using the single mode waveguide. An existing concept xe2x80x9celectromagnetic wave is radiated when i enclosed structure is lostxe2x80x9d generally infiltrated to waveguide engineers as common sense has now been reversed.
If a structure which can reproduce the foregoing physical phenomenon occurring around the reflection mirror, can be truly reproduced on the waveguide integrated circuit, the light beam can be bent to an arbitrary angle as if the reflection mirror is present.
For understanding of such phenomenon, it may be easier to understand action of the reflection mirror according to a principle of Huygens-Fresnel once returning to principle of wave analysis away from concept of waveguide mode. By this, when a beam form wave incides to the reflection mirror having a certain size of opening, it can be understood that infinite number of elemental waves with taking the arbitrary point on the reflection mirror as a wave source, is generated.
Real mode of the beam shaped reflected wave observed in macro, is a main lobe obtained by combining complex amplitude of all of elemental wave and can appear as a sharp beam shape having quite narrow solid angle of the main lobe and restricted the side lobe only when the opening of the reflection mirror is sufficiently large in comparison with the wave length. The propagating direction of the wave surface of the reflected wave (equiphase surface) is varied the orientation for a certain angle with respect to the wave surface of the incident wave. A relationship of the angle has been widely known as a law of incident angle and discharge angle.
In the process where the wave varies own propagation path by presence of the reflection mirror, it is nothing more than that an interference effect of the wave in the multi-mode space is interposed for bending the wave surface (using the multi-mode space). In this case, naturally, the wave may not change the propagation path unless the wave surface is deflected. Namely, the corner reflector as typical waveguide bending element may not cause mutual action efficiently with the multi-mode space as long as it is mounted on the single mode waveguide. As a result, the function as the pure reflection mirror cannot be achieved efficiently.
In the foregoing explanation, it can be expected that the multi-mode space corresponding to a free space around the reflection mirror and a radiation structure for efficiently cause mutual action (coupling) between the multi-mode space and the single mode waveguide upon bending the light at arbitrary angle.
Then, discussion will be given for a structure to provide the single mode waveguide for efficiently exchanging energy of wave between the multi-mode space (radiation and reception).
As can be clear from reciprocal theorem, a transmitting antenna and a receiving antenna are equivalent except that a time development of mutually acting electromagnetic wave is opposite. This means that the wave radiated from the antenna can be completely received by employing a certain structure in a relationship of the transmission antenna and the mirror image with respect to the equiphase surface (wave surface) of this wave.
By this principle, as shown in FIG. 2, two antennas 8 and 9 having 45xc2x0 of radiation directionality, for example, and the equiphase surface (wave surface) of the radiated wave become planar, are arranged to be mutually orthogonal. Then, complete couple of energy between these two antennas 8 and 9 should be possible.
Namely, by a combination of a pair of symmetric antennas, bending of 90xc2x0 can be equivalently realized. The present invention can provide means for realizing a waveguide discontinuity of large bending angle on the basis of principle of coupling between the waveguide utilizing the leaky wave.
As a structure having properties of both of antenna and the single mode waveguide, the leaky wave waveguide (called as leaky wave antenna in viewpoint of antenna researcher) has been known. The leaky wave is a phenomenon causing a given rate of radiation to a certain particular direction in a slow-wave region (region propagating through the waveguide with respect to a effective refractive index greater than the effective refractive index of the single mode waveguide) in the vicinity of certain waveguide.
This can be considered as extreme making an adjacent distance infinitely smaller between respective antenna elements of the antenna group (phased array antenna arranged in an array fashion). Namely, upon incident to the reflection mirror wave, an equivalent condition as a condition where infinite number of elemental waves are generated on the surface.
On the other hand, the leaky wave propagation region also serves as the multi-mode space necessary for avoiding mutual interference of the elemental waves generated around the reflection mirror. Furthermore, the equiphase surface of the radiated wave is planar and, as a result, is relatively sharp in the radiation directionality.
Particularly, since the equiphase surfaces of the waves are planar, by preparing a structure reflected with taking the equiphase surface as a plane of symnmetry, ideal coupling of energy between two symmetrical leaky structure can be established. Thus, it is expected that in the waveguide discontinuity where the straight waveguide is bent or intersected, the energy of the wave launched from the incident end of one wave guide can be efficiently taken out from the output end of the other waveguide. The leaky wave waveguide is ideal structure in realization of acute waveguide discontinuity.
On the other hand, between the effective propagation wavelength xcexG (effective refractive index nG) of two single mode waveguides, the narrower intersection angle xcex8 (0 less than xcex8xe2x89xa690xc2x0) at the intersecting point of the center axes extending in the longitudinal direction, and the effective propagation wavelength xcexL of the leaky wave propagation region, the following relationship is established:
cos(xcex8/2)=xcexL/xcexG
xe2x80x83=(xcexO/nL)/(xcexO/nG)
xe2x80x83=nG/nL
This expression is just Snell""s Law describing diffraction of the wave in a boundary surface where two regions respectively having effective refractive index of nG and nL being in contact. On the other hand, the structure taking the wave surface of the leaky wave as an axis of symmetry can be easily realized with the structure shown in FIG. 2.
The waveguide discontinuity in the two-dimensional planer waveguide integrated circuit is considered as most effective application of the present invention. For realizing this, it becomes necessary to appropriately design the refractive index of the material and dimensions of the structure so that optimal effective refractive index can be obtained in respective of the single mode waveguide or the leaky wave propagation region.
For necessity of selection of large effective refractive index, the material having large refractive index having the leaky wave propagation region has to be selected inherently. Therefore, possibility of causing multi-mode in the layer thickness direction of the substrate is concerned. It is desired to make the number of waves in the layer thickness direction of the leaky wave propagation region into the single mode and to make it consistent in the layer thickness direction of the single mode waveguide, as long as possible.
It should be noted that a realizing method of acute waveguide discontinuity using the leaky wave coupling discussed herein is not limited to the waveguide structure of the electromagnetic wave, such as extremely high frequency wave, light or the like. It is applicable for the waveguide discontinuity for guiding the electron, carrier injection from the carrier layer to the well layer in a lower order quantum well structure of a compound semiconductor and so forth, which are inherent upon fabrication of a quantum effect device utilizing electron required to be handled as de Broglie wave, for example.
Furthermore, analyzing the coupling between the waveguide using the leaky wave in another viewpoint, the coupling phenomenon of the leaky wave can be obtained by quite simple combination of vectors which can be expressed by:
xcexaL=xcexa1+xcexa2
assuming the wave number vector of the incident side single mode waveguide is xcexa1 and the wave number vector of the leaky wave propagation region is KL and the wave number vector of the output side single mode waveguide is xcexa2.
As a result of this, as the leaky wave propagation region generating the wave number vector xcexaL, possibility of use of a method other than the method to increase the effective refractive index, namely possibility of replacing with the period structure of the diffraction grating or the like providing the wave number vector equal to xcexaL.
In general, forming the regions having significantly differentiated the effective refractive indexes on the same substrate, cannot be said to be easily achieved in view of problem in complexity of the fabrication process or difficulty in establishing alignment between the waveguide materials. When application of the optical waveguide integrated circuit is concerned, the later finding that the period structure can be effective.